Atlas of natural and induced fractures in core PDF
Preview Atlas of natural and induced fractures in core
Atlas of Natural and Induced Fractures in Core Atlas of Natural and Induced Fractures in Core John C. Lorenz FractureStudies LLC New Mexico, United States Scott P. Cooper FractureStudies LLC New Mexico, United States This edition first published 2018 © 2018 John Wiley & Sons Ltd All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions. The right of John C. Lorenz and Scott P. Cooper to be identified as the authors of this work has been asserted in accordance with law. 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Neither the publisher nor authors shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. Library of Congress Cataloging‐in‐Publication data applied for ISBN: 9781119160007 [hardback] Cover Design: Wiley Cover Image: Photos by Scott P. Cooper Set in 10/12pt Warnock by SPi Global, Pondicherry, India 10 9 8 7 6 5 4 3 2 1 Dedication For my family, always there for each other, extraordinary. Scott P. Cooper For Bridget and Jackson, and the vast potential of a child’s unknown future. John C. Lorenz vii Contents Foreword xi Preface xiii Acknowledgments xv Introduction 1 Part 1 Natural Fractures 9 Section A Extension Fractures 11 A1 High‐Angle Extension Fractures 13 A1a Introduction 13 A1b Fractography of High‐Angle Extension Fractures 14 A1c Extension Fracture Dimensions 21 A1d Extension Fracture Variations and Lithologic Influences 33 A1e High‐Angle Extension Fracture Intersections 43 A1f High‐Angle Extension Fractures in Deviated Core 47 A2 Inclined Extension Fractures 51 A2a Inclined Extension Fractures in Horizontally Bedded Strata 51 A2b Inclined Extension Fractures in Inclined Strata 53 A2c Vertical Extension Fractures in Inclined Strata 54 A3 Horizontal Extension Fractures 55 A3a Beef‐Filled Fractures 55 A3b O ther Calcite‐Mineralized Horizontal Extension Fractures 57 A3c N OT Horizontal Extension Fractures 57 Section B Shear Fractures 59 B1 Introduction 61 B1a Nomenclature 61 B1b Anderson’s Shear Fracture/Fault Classification 62 B2 Shear Fracture Dimensions 65 B3 Shear Fracture Fractography 67 B3a S lickensides, Slickenlines, and Accretionary Steps 67 B3b E n Echelon Segments 69 B3c Steps 71 B3d P inch and Swell 72 B3e S heared and Glassy Surfaces 75 viii Contents B3f Slickencrysts 75 B3g Other Evidence for Shear 76 B4 High‐Angle Shear Fractures 79 B4a I ntroduction 79 B4b High‐Angle Strike‐Slip Shear Fractures 79 B4c Non‐Ideal High‐Angle Shear Fractures 82 B5 Intermediate‐Angle Shear Fractures 85 B6 Low‐Angle Shear Fractures 89 B7 Bed‐Parallel Shear Fractures 93 B8 Deformation Bands 97 B9 Faults 101 Section C Other Types of Natural Fractures 105 C1 Introduction 107 C2 Microfractures 109 C3 Ptygmatically Folded Fractures 111 C4 Fissures 117 C5 Veins 119 C6 Expulsion Structures 121 C7 Syn‐Sedimentary Fractures 125 C8 Compound/Reactivated Fractures 133 C9 Shattered Rock 137 C10 Karst Breccias 139 C11 Pocket‐Size Geomechanical Systems 143 C12 Stylolites 147 Section D Mineralization 151 D1 Mineralization 153 D1a Introduction 153 D1b Calcite Mineralization 154 D1c Other Types of Mineralization 160 D1d Oil and Bitumen 163 D1e False Mineralization 167 Contents ix Part 2 Induced Fractures 171 2A Introduction 173 2B Petal and Saddle Fractures 175 2C Centerline Fractures 185 2D Disc Fractures 197 2E Scribe‐Knife Fractures 209 2F Torque and Helical Twist Fractures 213 2G Core‐Compression Fractures 219 2H Percussion‐Induced Fractures 221 2I Bending Fractures with Barbs 225 2J Irregular Crack Networks 229 2K Induced Fractures with Curved Strikes 233 2L Waterflood‐Related Fractures 237 2M Cored Hydraulic Fractures 241 Part 3 Artifacts 245 3A Introduction 247 3B Core Tops and Core Bases 249 3C1 Core‐Catcher Drag 253 3C2 Core Orientation Scribe Grooves 257 3C3 Irregular Core Diameters 261 3C4 Pinion Holes 263 3D1 Spinoffs 265 3D2 Twice‐Turned Core 269 3E Saw Scars 271 3F1 Core Plucking 277 3F2 Scratches 279 3F3 Drill‐Mud Erosion 281 x Contents 3F4 Core‐Parting Enigmas 283 3F5 Polished Fracture Surfaces in Horizontal Cores 285 3F6 Tip Polish 287 3F7 Slab‐Plane Consistency 291 3F8 Illusions 295 3F9 Coring‐Related Rock Alteration on Core Surfaces 299 Index 301 xi Foreword Fractured reservoirs are a significant part of our their distribution and character can be interpreted to hydrocarbon production across the world, even though determine the causative forces that generated them. most were discovered to be fracture-dominated after The reasons why we make these determinations are initial drilling. These fractured reservoirs are some of the to predict fracture distributions and characteristics for largest and most productive oil and gas fields world‐ future exploration and development wells and to con- wide. Over the last three decades, we have advanced in strain numerically a Static Conceptual Fracture Model our understanding of these reservoirs and how to model in reservoir simulation. the fracture systems quantitatively and create a Static Previous catalogs of fractures in rock have been pub- Conceptual Fracture Model. Today, using the Static lished in book format by Kulander et al. (1990) and in Conceptual Fracture Model as input in reservoir simula- numerous industry‐related course notes covering frac- tions allows us to explain previous production behavior tured reservoirs by both Lorenz and Cooper and myself. and to predict future reservoir response. These models However, this volume goes far beyond the previous require compilation and integration of numerous data versions in terms of completeness. It is a testament to types derived from multiple observations and discipline the vast experience of the co‐authors in working with sources. fractures in core. I can think of none better to organize This atlas by Lorenz and Cooper looks exclusively at and create this atlas. one of these observational sources, that of core evalua- The authors have extensive experience over several tion. For many studies, this is the “ground truth” of sub- decades in describing and quantifying fractures in out- surface fracture characteristics. However, what we see in crop and core. Much of this experience has been in direct the core from the perspective of fractures is a mix of support of oil and gas company operations and industry/ what is in the subsurface and what we have done to the academia consortia as well. Lorenz and Cooper have core during and after the coring process. These “arti- seen it all and understand how to interpret it. facts” must be omitted from our descriptions as not nat- I believe that this volume is the ultimate resource for urally occurring. I believe that a strong point of this atlas “reading the story and history” of fractures in rocks from is in learning how to make this distinction. core. It is a “must‐have” volume for all who have, or wish Using the examples displayed in this volume, we can to have, an intimate knowledge of the rocks they work determine which fractures are naturally occurring and with from a fracture point of view. which are induced due to effects of coring, core han- dling, weathering, and stress relief, among others. The R.A. Nelson remaining fractures are considered natural fractures and Broken N Consulting, Inc. xiii Preface This book describes the characteristics of individual few fractures captured from such a small sampling of a fractures as they are expressed in cores cut from hydro- reservoir, as well as to correctly interpret those fractures. carbon reservoirs, and it provides criteria for distin- The fractures described here are the building blocks guishing the various fracture types as they are expressed for the fracture networks and systems that influence in the limited fracture samples intersected by those reservoir permeability. Discussions and descriptions of cores. Different natural fracture types have different those three‐dimensional systems or of their behavior effects on reservoir permeability, and induced types have within a changing stress field as a reservoir is stimulated little or no effect at all, so it is imperative to correctly or produced are not part of this volume. Neither do we interpret cored fractures if the goal is ultimately to discuss fracture mechanics and some of the issues sur- understand the broader fracture system and its influence rounding the origin of fractures, although some of our in enhancing or degrading reservoir permeability. prejudices are evident. The butts and slabs from a 30‐ft long, 4‐inch diameter Rather, we present here examples of the wide range of core comprise a significant amount of material when laid fracture types and characteristics that influence reser- out in the lab for detailed examination, but volumetrically voir permeability so that they can be recognized in cores this is a miniscule sampling of a reservoir. Thus it is imper- and used to construct both conceptual and numerical ative to maximize the amount of data obtained from the models of reservoirs.
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